Nickel hydrogen and sealed nickel cadmium batteries have been used in spacecraft applications and have been designed to operate in an electrolyte starved configuration where there is no free liquid electrolyte within the cell. All of the electrolyte is contained by capillary forces within the pores of the electrodes, wall wick, and separators within the cell. This starved configuration simultaneously enables uniform transport of both gases and liquid electrolyte within the electrode stack and in the gas spaces surrounding the electrochemical components within the cell. An excessively starved cell will perform poorly because the separators having large pores will become dry, making the cell unable to support high rates of ionic current flow through the separators.
At the other design extreme, an excessively flooded configuration will not allow uniform transport of gases within the cell. For nickel hydrogen cells, the flooded configuration can result in two problems with gas transport. The first problem occurs when some areas of the hydrophobic side of the negative plates become flooded with electrolyte to limit the accessibility of hydrogen gas to the platinum catalyst in the negative electrode. The second problem occurs when free electrolyte is present in the regions through which oxygen must flow as the oxygen escapes the electrode stack during overcharge. Bubbles of high-pressure oxygen can accumulate in such regions of free electrolyte. These bubbles of oxygen, when contacting the platinum catalyst, can ignite to cause small explosive thermal popping events. Such popping events can occur either over the surface of the negative electrode, or at the edges of the negative electrodes where large amounts of oxygen can be channeled to the edges of the negatives. In the back-to-back stack design of large nickel hydrogen cells, popping at the edges of the plates is generally the more significant. Significant popping events can result in cell short circuits as a result of damage to the edges of the plates or separators.
Ground life-test cycling of nickel hydrogen cell designs can be very misleading in identifying popping problems during prospective spacecraft usage as a result of excessive electrolyte. Cells are typically tested in a vertical configuration that gravitationally drains all free electrolyte into a pool in the bottom of the cell case. Alternatively, testing cells on their sides has been found to also not represent the zero-gravity environment of space because the electrolyte tends to settle towards the downwards side of the electrode stack. Horizontal life testing has often led to early failures due to popping problems, although horizontal life testing also represents a worst-case stress condition for popping problems. Popping damage can lead to short circuits and failures of the battery cells. These and other disadvantages are solved or reduced using the invention.